feat(nix): expose NixOS module and add CLI args for host/port

- Expose nixosModules.default in flake outputs for easy import
- Add argparse to main.py for --host and --port CLI flags
- Support priority: CLI args > env vars > defaults

flake.nix

@@ -18,8 +18,11 @@
     flake-utils,
     m3ta-nixpkgs,
     agents,
-  } @ inputs:
+  }:
-    flake-utils.lib.eachDefaultSystem (system: let
+    {
+      nixosModules.default = import ./nix/module.nix;
+    }
+    // flake-utils.lib.eachDefaultSystem (system: let
       pkgs = nixpkgs.legacyPackages.${system};
       pythonPackages = pkgs.python311Packages;
 

src/main.py

@@ -1,8 +1,10 @@
 """FastAPI application for ZUGFeRD invoice processing."""
 
+import argparse
 import base64
 import json
 import logging
+import os
 from datetime import datetime
 
 import uvicorn
@@ -147,8 +149,24 @@ async def validate_invoice_endpoint(request: ValidateRequest) -> ValidateRespons
 def run(host: str = "0.0.0.0", port: int = 5000) -> None:
     """Run the FastAPI application.
 
+    Priority: CLI args > env vars > defaults
+
     Args:
-        host: Host to bind to.
+        host: Host to bind to (default from env HOST or 0.0.0.0).
-        port: Port to listen on.
+        port: Port to listen on (default from env PORT or 5000).
     """
-    uvicorn.run(app, host=host, port=port)
+    parser = argparse.ArgumentParser(description="ZUGFeRD REST API Service")
+    parser.add_argument(
+        "--host",
+        default=os.environ.get("HOST", host),
+        help="Host to bind to (default: 0.0.0.0)",
+    )
+    parser.add_argument(
+        "--port",
+        type=int,
+        default=int(os.environ.get("PORT", port)),
+        help="Port to listen on (default: 5000)",
+    )
+    args = parser.parse_args()
+
+    uvicorn.run(app, host=args.host, port=args.port)